In addition to radiant intensity, important parameters of light sources are the hue thereof and the colour saturation thereof. Precise adjustment of the latter two parameters is necessary in many applications, for example in light sources which are designed to replicate natural daylight as closely as possible. Apart from these white light sources, light is often required in just one specific spectral range, for example for projection purposes or for coloured display devices such as for instance advertising hoardings.
Typically, two different approaches are adopted in order to obtain light sources with the desired hue and the desired colour saturation. In the first case a light source is used which already emits light with the desired colour characteristics. Although radiation emitters emitting in different colours are technically producible in particular in the field of semiconductor light emitting diodes, purposeful adjustment of the colour is often possible only to a limited extent and may additionally be very complex. In the case of broadband-emitting white light sources such as for instance light bulbs, which may be regarded as black bodies, the achievable range of parameters for colour is limited physically by the thermal loading capacity of the materials used, while the “colour temperature” may be influenced only to a limited extent. In the second case the desired spectral range is achieved by way of a conversion medium, which modifies the original wavelength of the light emitted by the source. Conversion media or luminescent materials must satisfy stringent requirements with regard to efficiency or quantum yield and to thermal, light or chemical stability, must as a rule be inexpensive to produce and also easy to fix to the light source. Achieving all these characteristics, in particular purposeful tuning of the spectrum emitted by the conversion medium, is often difficult, complex and costly. However, there are often limits to what can be achieved.
Colour filters are also often used, which only transmit light in a specific wavelength range and so influence hue and colour saturation. Filter media or absorbers are usually much easier to tailor to requirements with regard to colour properties than are conversion media. However, the use of filters is as a rule associated with losses in terms of light intensity. Since, however, considerable importance is attached to the efficiency of a light source, filter media are typically not regarded as the ideal solution.